Magnetic recording mediums are widely used in the fields of recording tapes, video tapes, computer tapes and disks. In the field of the magnetic recording mediums, higher recording density has been demanded, and therefore recording wavelengths have been shortened. The recording systems ranging from the analog system to the digital system have been examined. Magnetic recording mediums using a thin metallic film as the magnetic layer have been proposed to meet the requirements of higher recording density. Coated type magnetic recording mediums prepared by coating a binder containing ferromagnetic powder dispersed therein, on a support are superior in productivity and corrosion resistance.
However, the degree of loading of the ferromagnetic powder in the coated type magnetic recording mediums is low in comparison with the thin metallic film type magnetic recording mediums. Hence the coated type magnetic recording mediums are inferior in electromagnetic characteristics.
Coated type magnetic recording mediums comprising a non-magnetic support having thereon a magnetic layer composed of a ferromagnetic substance such as ferromagnetic iron oxide, Co-modified iron oxide, CrO.sub.2 or a ferromagnetic alloy powder dispersed in a binder, are widely used. Various methods have been proposed to improve the electromagnetic characteristics of the coated type magnetic recording mediums. For example, attempts have been made to improve the magnetic characteristics of the ferromagnetic powder or to smoothen the surface of the magnetic layer. However, the requirements for higher recording density cannot be sufficiently met.
Recording wavelengths have been shortened and recording density has been improved in recent years. However, when the thickness of the magnetic layer is increased, output is lowered. Problems with regard to self-demagnetization loss during recording and thickness loss during reproduction, become serious.
Therefore, in order to solve the above problems, the thickness of the magnetic layer has been reduced. However, when the thickness of the magnetic layer is reduced to about 2 .mu.m or less, the surface of the magnetic layer is likely to be affected by the surface roughness of the non-magnetic support. Further, the electromagnetic characteristics and DO (drop out) may deteriorate.
The effect of the surface roughness of the non-magnetic support on the surface of the magnetic layer can be improved to some degree by providing a non-magnetic thick undercoat layer on the surface of the support, and providing thereon a magnetic layer as the upper layer as described in JP-A-57-198536 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, the degree of the improvement is insufficient, and problems exist in that head friction and durability cannot be improved. This may be caused by the fact that a thermosetting resin as a binder is conventionally used in the non-magnetic lower layer, the lower layer is cured, and the friction between the magnetic layer and head, or the contact of the magnetic layer with other members, is made in a non-cushioning state. Alternatively, it may be that the magnetic recording medium having a lower layer as described above is slightly poor in flexibility. It can be considered that a non-curing resin such as a binder is used in the lower layer to solve the above-described problems. In conventional methods, however, when the magnetic layer as the upper layer is coated after the coating and drying of the lower layer, problems result such as the lower layer swelling by an organic solvent contained in a coating composition for the upper layer and a turbulent flow forming in the coating solution for the upper layer. Aa a result, the surface properties of the magnetic layer are deteriorated and electromagnetic characteristics are lowered.
The thickness of the magnetic layer can be reduced by decreasing the coating weight of the coating composition or lowering the concentration of the magnetic coating composition by adding a large amount of a solvent to the magnetic coating composition. In the former case, where the coating weight of the coating composition is decreased, the time required for leveling after coating is shortened so that when the coating composition begins to dry a failure in coating occurs. For example, stripes or marked patterns are left behind, and yield is very low. In the latter case, where the concentration of the coating composition is lowered, there are the disadvantages that the concentration of the magnetic coating composition is dilute, many voids are formed in the resulting coating film and a sufficient degree of the loading of the magnetic substance cannot be attained, and when many voids are formed, the strength of the coating film is insufficient. Poor yield is a serious problem as described in JP-A-62-154225.
U.S. Pat. No. 4,963,433 (corresponding to JP-A-63-191315) and U.S. Pat. No. 4,863,793 (corresponding to JP-A-63-187418) propose a magnetic recording medium having improved electromagnetic characteristics by using a simultaneous multi-layer coating system as a means for solving the above-described problems. However, the following problems are not solved by this system.
Namely, when the thickness of the magnetic layer is reduced to as thin as 1 .mu.m or less, a failure in coating occurs. For example, pinholes and stripes are likely to form. Therefore, productivity is hard to ensure.
Further, it is demanded to provide a magnetic recording medium having very smooth surface properties. Calendering, in particular, must be carried out under high temperature and high pressure conditions to obtain very smooth surface properties. When calendering is carried out under high temperature and high pressure conditions, the friction between the opposite side of the support to the magnetic layer side, and the calender roll is increased, this results in the support being cut during the preparation of the magnetic recording medium, and the calender rolls become contaminated. When the rolls are contaminated, contaminated marks are left on the magnetic layer after calendering and as a result, a lowering in electromagnetic characteristics and an increase in drop out occur.
A magnetic recording medium comprising a polyethylene terephthalate base having thereon a lower magnetic layer (first magnetic layer) and an upper magnetic layer (second magnetic layer) in this order, wherein the polyethylene terephthalate base contains inert inorganic particles having an average particle size of 0.5 to 0.7 .mu.m and such a particle size distribution that the standard deviation thereof is 0.1 .mu.m or less, and wherein the polyethylene terephthalate base has a center line average roughness (Ra) of 16 nm or less and has protrusions of 0.7 .mu.m or above at a protrusion density (Ds) of at least 6,000 protrusions/mm.sup.2 has been proposed in order to provide a magnetic recording medium with high productivity which has a magnetic layer having excellent electromagnetic characteristics provided on the base having a smooth surface and moreover, which has good running durability on the back side without providing a back coat layer. However, the degree of the improvement is still insufficient merely by decreasing Ra. U.S. Pat. No. 5,116,658 (corresponding to JP-A-3-176809) proposes the reduction of undulation on the surface of the magnetic layer and the interfacial undulation of the support. However, the improvement of electromagnetic characteristics are still insufficient merely by reducing undulation, and protrusions in the amount of 0.30 .mu.m (300 nm) in height present on the opposite side of the support to the magnetic layer is small so that unevenness is distinct and running properties are poor.
JP-A-3-86916 discloses a non-magnetic support having a two-layer structure which is a multi-layer non-magnetic support, wherein the lower layer thereof contains a given amount of filler particles having a given particle size. When the magnetic recording medium having such a support as described above is wound up, the magnetic layer side and the back side of the support are brought into contact with each other, and various recesses are formed on the surface of the magnetic layer by protrusions formed by the filler particles. As a result, the electromagnetic characteristics are lowered by the formation of recesses. The costs of mechanical parts used in the latest VHS video cassette have been reduced. Hence, scuff resistance to so-called inferior pins, etc. becomes poor and as a result, problems with regard to DO increase, a failure in running, etc. are caused.
The present inventors have eagerly made studies to solve the above-described problems by paying attention to layer structures such as the surface roughness of the non-magnetic support and the state of the protrusions and the magnetic layer provided on the support. As a result, the present inventors have found that the center line average roughness Ra varies with the cut off values, and a remarkable improving effect can be obtained by taking into consideration a combination of the difference in Ra between the cut off values with a plurality of layers formed by providing an upper magnetic layer on a lower non-magnetic layer or a lower magnetic layer by means of a wet-on-wet (simultaneous or successive) coating system. The present invention had been accomplished on the basis of this finding.